today: moon phases - the university of arizonajweirich/ptys_206_2/lectures/lec5-012507.pdf ·...

PTYS/ASTR 206 Lunar Phases / Eclipses1/25/07

Today:Moon Phases


Announcements

• The first homework due now.– The next assignment is now posted

• due Tuesday, Feb. 6

• Weekly preceptor-led study group– Every Monday 10:30AM-12:00Noon


Today’s topics

• What causes moon phases?• What is Earthshine?• What causes eclipses?


The Phases of the Moon• Key Points:

– The Moon, at any time, is ½ illuminated by the Sun. The sunlit side always faces the Sun.

– The Moon orbits the Earth. As it does so, we see different fractions of the illuminated side.

Sunlight


Nomenclature• The “Dark Side” of the Moon

– This is the side that is not illuminated by the Sun.– Note that this side of the Moon often faces us (a

New Moon, for instance)

• The “Far Side” of the Moon– This is the side of the Moon that is not facing the

Earth– We never see this side from Earth.

• Note that the Dark Side and Far Side are NOT the same – except during a full moon ONLY


The New Moon• When the Moon is between

the Earth and Sun – it is in the New Moon phase

• The New Moon rises at sunrise, sets at sunset, and is highest in the sky at noon (just like the Sun!)

• This is the best time to observe the stars in the night sky without the glare of the moon interfering –dark skies


• The New Moon is the only time that a solar eclipse can occur


The Full Moon• When the Moon is at opposition –

i.e on the opposite side of the Sun, we see the fully illuminated side. This is the Full Moon phase

• The Full Moon rises at sunset, sets at sunrise (exactly opposite to the Sun)

• This is the worst time to observe the stars in the night sky because of the Moon’s glare

– It is also not an ideal time to observe features on the moon through a telescope.


• The Full Moon phase is the only time in which a lunar eclipse can occur


The First-Quarter Moon• When we see half of the sunlit side of

the Moon it is said to be a quarter moon. If this occurs before the Full Moon (and after the New Moon) – the phase is the First-Quarter Moon

• The First-Quarter Moon rises at noon, and sets at midnight.

• This is an excellent time to observe Moon features such as craters, rillesand rimae – it is best to look along the terminator (the separator between light and dark)– Go out shortly after sunset to get the

best views (it is highest in the sky)


The Third-Quarter Moon

• If a quarter-moon phase occurs after the Full Moon (and before the New Moon) – the phase is the Third-Quarter Moon

• The Third-Quarter Moon rises at midnight, and sets at noon.

• Just like the 1st-quarter Moon, this is an excellent time to observe moon features though a telescope.– Go out just before sunrise (it is

highest in the sky)


Waxing and Waning phases

• Waxing (“increasing”) is the term used to describe the phases after the New Moon and before the Full Moon– During this phase, each day we see

more and more of the illuminated side of the Moon

– Includes the first-quarter phase– The beautiful crescent Moon seen

just after sunset is in a “waxing” crescent phase

• Waning (“decreasing”) is the term used to describe the phases after the Full Moon and before the New Moon– During this phase, each day we see

less and less of the illuminated side of the Moon

– Includes the third-quarter phase– The crescent Moon seen just before

sunrise is in a “waning” crescent phase

This is an example of a “waxing” moon(it increases with time)

May 3 May 1 April 29


Gibbous and Crescent

• Gibbous means the “full”-ishphase which is when the Moon is between the 1st and 3rd

quarter phases and we see more than half of the illuminated side of the Moon

• During the crescent phases we see less than half of the illuminated side of the Moon

Gibbous Moon

CrescentMoon


Order of the Moon Phases

(1. - start) New Moon – dark skies !– Moon is not visible (glare from

the Sun)

(2. - after a few days)– Waxing crescent– Sets shortly after sunset

(3. - about 7 days) First Quarter Moon– Actually a 1/2 moon– Moon goes from a crescent to a

“gibbous” moon

(4. - about 14 days) Full Moon– Goes from waxing to waning

(5. - about 21 days) Third-Quarter moon– Another ½ moon– Waning phase– Goes from gibbous to

crescent

(6. - about 26 days)– Waning crescent– Rises shortly before sunrise

(1. - start again) New Moon


What is a Blue Moon?• Two definitions:

(1) The second full moon within one calendar month

(2) the 3rd full moon in a season that has 4 full moons (huh?)

– From the Farmer’s Alamanc!– The 4th is the “Harvest Moon”

• It is NOT BLUE !!


Today’s topics



Earthshine

Sunlit side of the Moon

Dark side of the Moon,Illuminated by Earthshine


• Earthshine is due to the double reflection of sunlight.– Light from the Sun is reflected off of the Earth (point A) onto the

Moon (point B). Some of this light is then reflected off of the Moon back towards the Earth (point C).

• The brightest part of the Moon is illuminated by the Sun and the rest is dimly illuminated by this doubly reflected light.


Saturn’s moon Iapetus seen in “Saturnshine”


Today’s topics



Eclipses• Lunar Eclipses

– This occurs when the Earth “shades” the Moon as it passes between the Moon and Sun

– Total, Partial, Penumbral– Occurs only during a full moon

• Solar Eclipses– This occurs when the Moon

passes in front of the Sun, blocking the Sun from view from Earth

– Annular, Total– Occurs only during a new

moon Lunar Eclipse of 10/27/04 photo by Instructor


Lunar Eclipses• Does the moon

become invisible?– No, refraction of

sunlight through the Earth’s atmosphere causes the moon to become reddish


Lunar Eclipse


Solar Eclipses


Total solar eclipse


Annular Solar Eclipse


Eclipses occur only when the Sun and Moonare both on the line of nodes


Geometry and Path of Totality


Eclipse Paths


Reading for next class

• Chapter 5– pp. 90-114 (you can omit section 5-8)


Extra slides


Guiding questions/concepts

• What causes moon phases?• What is Earthshine?• What is the difference between the

synodic and sidereal month?• What causes eclipses?


Time and the Moon• Two types of months

are used in describing the motion of the Moon.

• With respect to the stars, the Moon completes one orbit around the Earth in a sidereal month, averaging 27.32 days.

• The Moon completes one cycle of phases (one orbit around the Earth with respect to the Sun) in a synodicmonth, averaging 29.53 days.


Guiding questions/concepts

• What causes moon phases?• What is Earthshine?• What is the difference between the

synodic and sidereal month?• What causes eclipses?

today: moon phases - the university of arizonajweirich/ptys_206_2/lectures/lec5-012507.pdf ·...

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